asyncio.subprocess blocked event loop unexpectedly on MacOS

Bug report

Bug description:

After upgrading to 3.14.7 & macOS 27, this bug has happened.

When pausing the subprocess created by asyncio.create_subprocess_exec, it will blocked the whole event loop on MacOS.

Here is a minimal test code:

import asyncio
import psutil

async def main():
    proc = await asyncio.create_subprocess_exec(
        "sleep",
        "1000",
    )
    psutil.Process(proc.pid).suspend()

    print("child pid:", proc.pid, flush=True)

    count = 0
    while True:
        await asyncio.sleep(1)
        print(f"alive {count}", flush=True)
        count += 1

asyncio.run(main())

Expectly, it should print:

child pid: xxxxx
alive 0
alive 1
...

However, it just prints child pid: xxxxx, without any alive.

This issue isn't caused by psutil because the same result was got with kill -STOP xxxxx.

This runs well on Windows, but not on MacOS. This didn't happen on the previous version of 3.14.

CPython versions tested on:

3.14

Operating systems tested on:

macOS, Windows

Linked PRs

gh-158287: Fix blocked asyncio event loop on macOS when child process is stopped #158962

@kumaraditya303

cc @kumaraditya303

@starstreammm

@starstreammm Do you mean that on Python 3.14.6 it works well? or is it on 3.13 that it works well? (this is to confirm whether it's a release blocker or not)

@starstreammm Do you mean that on Python 3.14.6 it works well? or is it on 3.13 that it works well? (this is to confirm whether it's a release blocker or not)

I updated my Python version yesterday. It had been quite a while since my last update, but I can confirm that I was already using 3.14 after the update. This issue didn't happen on that version. But I don't remember which version I used before exactly.

I'm going to let Kumar have a look at this issue and decide whether this is a release-blocker. At least, it's a regression AFAICT.

hey, i just took a quick look with my own tooling (on macos too)

The trace:

[EventLoopBlock] 'main' blocked the loop for 0.735s (threshold: 0.1s)
Stack at the moment of blocking:
  File "/Users/timofeiivankov/cpython/repro.py", line 199, in <module>
    asyncio.run(main())
  File "/Users/timofeiivankov/cpython/Lib/asyncio/runners.py", line 205, in run
    return runner.run(main)
  File "/Users/timofeiivankov/cpython/Lib/asyncio/runners.py", line 128, in run
    return self._loop.run_until_complete(task)
  File "/Users/timofeiivankov/cpython/Lib/asyncio/base_events.py", line 712, in run_until_complete
    self.run_forever()
  File "/Users/timofeiivankov/cpython/Lib/asyncio/base_events.py", line 683, in run_forever
    self._run_once()
  File "/Users/timofeiivankov/cpython/Lib/asyncio/base_events.py", line 2090, in _run_once
    handle._run()
  File "/Users/timofeiivankov/cpython/Lib/asyncio/events.py", line 135, in _run
    return super()._run()
  File "/Users/timofeiivankov/cpython/Lib/asyncio/events.py", line 91, in _run
    self._context.run(self._callback, *self._args)
  File "/Users/timofeiivankov/cpython/Lib/asyncio/unix_events.py", line 1003, in _reap_and_notify
    pid, returncode = self._reap(loop, expected_pid)
  File "/Users/timofeiivankov/cpython/Lib/asyncio/unix_events.py", line 1008, in _reap
    pid, status = os.waitpid(expected_pid, 0)

-> Culprit: _reap() at /Users/timofeiivankov/cpython/Lib/asyncio/unix_events.py:1008

So _reap is probably the cause, or the code near it. If no one objects, I can work on the PR

@deadlovelll

Can you bisect the faulty commit first? @deadlovelll

Can you bisect the faulty commit first? @deadlovelll

Faulty commit - 2875d1d (#153810). Fix is needed for main, 3.15 and 3.14

This seems like a macos kernel bug to me, dotnet is also hitting this quirk in dotnet/runtime#133930

I think the fix is to exclude CLD_STOPPED manually here to avoid blocking in waitpid.

pythongh-158287: Add timeout to test_stopped_child_does_not_block_eve…

pythongh-158287: Clarify user-visible behavior in NEWS entry

pythongh-158287: Simplify CLD_STOPPED handling in _ThreadedChildWatcher

pythongh-158287: Use watchdog timer in test_stopped_child_does_not_bl…

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asyncio.subprocess blocked event loop unexpectedly on MacOS #158287

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asyncio.subprocess blocked event loop unexpectedly on MacOS #158287

Description

Bug report

Bug description:

CPython versions tested on:

Operating systems tested on:

Linked PRs

Activity

picnixz commented on Sep 27, 2026

picnixz commented on Sep 27, 2026

starstreammm commented on Sep 27, 2026

picnixz commented on Sep 27, 2026

deadlovelll commented on Sep 27, 2026

picnixz commented on Sep 27, 2026

deadlovelll commented on Sep 27, 2026

kumaraditya303 commented on Oct 5, 2026

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